Floating Points

Concrete partly supports floating points:

• They cannot be inputs

• They cannot be outputs

• They can be intermediate values under certain constraints

As intermediate values

Concrete-Compile, which is used for compiling the circuit, doesn't support floating points at all. However, it supports table lookups. They take an integer and map it to another integer. It does not care how the lookup table is calculated. The constraints of this operation are such that there should be a single integer input, and it should result in a single integer output.

As long as your floating point operations comply with those constraints, Concrete automatically converts them to a table lookup operation:

from concrete import fhe
import numpy as np

@fhe.compiler({"x": "encrypted"})
def f(x):
    a = x + 1.5
    b = np.sin(x)
    c = np.around(a + b)
    d = c.astype(np.int64)
    return d

inputset = range(8)
circuit = f.compile(inputset)

for x in range(8):
    assert circuit.encrypt_run_decrypt(x) == f(x)

In the example above, a, b, and c are floating point intermediates. They are used to calculate d, which is an integer with a value dependent upon x , another integer. Concrete detects this and fuses all of these operations into a single table lookup from x to d.

This approach works for a variety of use cases, but it comes up short for some:

from concrete import fhe
import numpy as np

@fhe.compiler({"x": "encrypted", "y": "encrypted"})
def f(x, y):
    a = x + 1.5
    b = np.sin(y)
    c = np.around(a + b)
    d = c.astype(np.int64)
    return d

inputset = [(1, 2), (3, 0), (2, 2), (1, 3)]
circuit = f.compile(inputset)

for x in range(8):
    assert circuit.encrypt_run_decrypt(x) == f(x)

This results in:

RuntimeError: Function you are trying to compile cannot be converted to MLIR

%0 = x                             # EncryptedScalar<uint2>
%1 = 1.5                           # ClearScalar<float64>
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ only integer constants are supported
%2 = y                             # EncryptedScalar<uint2>
%3 = add(%0, %1)                   # EncryptedScalar<float64>
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ only integer operations are supported
%4 = sin(%2)                       # EncryptedScalar<float64>
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ only integer operations are supported
%5 = add(%3, %4)                   # EncryptedScalar<float64>
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ only integer operations are supported
%6 = around(%5)                    # EncryptedScalar<float64>
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ only integer operations are supported
%7 = astype(%6, dtype=int_)        # EncryptedScalar<uint3>
return %7

The reason for the error is that d no longer depends solely on x; it depends on y as well. Concrete cannot fuse these operations, so it raises an exception instead.